1. Field of the Invention
This invention relates to fittings for nuclear reactor fuel elements and, more particularly, to orifice rod assemblies that expose the ends of the fuel rods for inspection, and the like.
2. Description of the Prior Art
Nuclear reactors generally have a core that generates heat as a result of the fission processes which occur in a critical concentration of uranium within that core. Usually, in power reactor systems, this critical concentration of uranium is assembled by loading pellets or uranium dioxide into hollow tubes, or fuel rods. These loaded fuel rods are then grouped into relatively rigid sub-assemblies, or fuel elements, which are placed together within the reactor pressure vessel in order to form the reactor core.
In addition to the array of fuel rods, these fuel elements also have in many cases a number of other rods, of which control rods and "burnable poison" rods are typical. Usually, these control and "burnable poison" rods are distributed throughout respective fuel rod bundles in order to control the level of fission process activity and hence, the power output from the reactor core as well as to increase the useful life of the reactor core.
As a general matter, the control rods each are received within respective guide tubes, the group of rods for a particular fuel element being joined together at one end by means of a casting that is referred to as a "spider." This configuration gangs together all of the control rods in one fuel element to enable these rods to move as a single group in a direction that is parallel to the longitudinal axes of the fuel rods. The range of this ganged control rod movement, moreover, extends during reactor operation from fully inserted into the fuel element to fully withdrawn from the fuel element.
In contrast to these control rods, the "burnable poison" rods do not move relative to the fuel rods while the fuel element is in place within the reactor core. In a manner that is similar, however, to the control rods the "burnable poison" rods in a particular fuel element are joined together at one end by means of a cast metal "spider," or orifice rod assembly.
As the fissionable material within the fuel elements that comprise the reactor core is consumed, to obtain maximum core life it frequently is advisable from time-to-time to shift the relative locations of the individual fuel elements within the core. In these circumstances, it often is preferable to relocate fuel elements that accommodate control rods at a new position within the core in which the fuel element should not house control rods, but "burnable poison" rods instead or, perhaps, no rods at all. Similarly, fuel elements without control or "burnable poison" rods now might be ideally relocated in places in which one of these two rod types should be housed within the element. Naturally, fuel elements with "burnable poison" rods also might be subject to repositioning at a station in which control rods, or no rods of either type would be more suitable.
Because these three types of fuel elements require different fittings, relocation of partially used fuel elements is conducted on a less than optimum repositioning schedule to avoid the need to modify the now radioactive fuel elements to accomodate the particular configuration of control rod, "burnable poison" rod (or neither of these two rods) that characterizes the new fuel element section. On the other hand, if optimum repositioning is desired, necessary modifications to the fuel elements must be carried out in a cumbersome and expensive manner with remote handling equipment. Further in this respect, the empty control rod guide tubes in those fuel elements that do not have either control rods or "burnable poison" rods present special thermal problems. Typically, to remove heat from the reactor core, the void spaces within the core are filled with flowing, pressurized water. In this situation the empty guide tubes in these fuel elements tend to channel relatively cold water from the inlet to the reactor core through to the core coolant outlet. This colder water mixes with the hot water that is discharging from the core and thereby produces an undesirable decrease in the average temperature of the coolant that flows from the core.
Under the conditions of pressure, heat, radiation and high velocity water flow, the structural integrity of the individual fuel rods as well as the fuel elements into which they are grouped also is of fundamental importance. Accordingly, to retain the fuel rods within their relative positions in the respective fuel elements, sturdy cast metal "end fittings" are used to engage both of the ends of each of these fuel rods and the control rod guide tubes as well as at least one end of the "burnable poison" rods in those fuel elements which have this type rod.
The lower end of the entire group of fuel elements that constitute the reactor core usually is supported on a grid-like structure, or lower grid plate, that sustains the weight of the fuel elements against the force of gravity. This lower grid plate also distributes the pressurized coolant that flows into the reactor core in a manner that insures, insofar as it is possible, that there is a generally uniform temperature distribution within the core and that "hot spots" in the reactor core are largely eliminated.
Because the coolant flows upwardly during reactor operation at a substantial pressure and flow velocity, there is a tendency for the hydraulic forces to lift the fuel elements from the lower grid plate. To counter this effect the reactor also is provided with a grill-like upper grid plate. In this instance portions of the upper end fitting bear directly against metal pads that protrude downwardly from this upper grid plate, thereby engaging the fuel elements and retaining them in proper respective position against these hydraulic forces and the spring-loaded upper end fitting.
As a matter of sound engineering practice, moreover, it is customary to inspect each fuel rod at generally regular intervals during the operational lifetime of the reactor core. After reactor operation has commenced, the fuel rods become radioactive and hence these routine inspections necessarily must be conducted with remote handling equipment in suitably shielded conditions. To carry out these inspections, it has been the practice to shut down the reactor and withdraw from the core the fuel elements that contain the fuel rods which are to be examined. These fuel elements are subsequently dismantled and the fuel rods are inspected individually in a "sipping" can in which a radiation detector checks the radioactivity of a test fluid that flows from the can in order to identify a notably high level of radioactivity which is indicative of a ruptured, defective rod.
This rather tedious procedure of remote disassembly, inspection and reassmbly is required for a number of reasons, not the least of which is the presence of the massive cast metal "spider" for the control rods or the "burnable poison" rods, depending on the nature of the particular fuel element These monolithic castings block direct observation and alignment between the individual fuel rod ends and a collimated radiation detector, thereby preventing the detector from identifying specific defective fuel rods. Further in this respect, the cast control and "burnable poison" rod "spiders" are quite expensive. These "spider" fittings also require very careful inspection to guard against flaws and those other defects that are frequently encountered in complicated cast shapes which must operate in hostile environments.
In these circumstances there is a need for a less expensive but equally durable control and "burnable poison" rod "spider" that will not obstruct fuel rod inspection, but will expose the individual rods for examination without imposing a need to dismantle the entire fuel assembly. There is, of course, a further need for a more flexible control and "burnable poison" rod assembly that will permit optimum fuel element relocation with the reactor core without requiring specific modifications or producing undesirable thermal effects.